DCA18FA144

The airplane suffered a right main landing gear collapse during landing at the destination airport. The airplane sustained substantial damage to the right lower wing skin when it contacted the runway after the landing gear collapse. The crew stopped the airplane on the runway and an emergency evacuation was performed through three of the four doors on the airplane. The escape slide at the left forward door did not deploy or inflate due to the depletion of the gas charge in the reservoir. The reservoir depleted due to a leak in the valve assembly and was not caught during multiple inspections since installation of the slide assembly in the airplane. The landing gear cylinder fractured under normal landing loads due to the presence of a fatigue crack on the forward side of the cylinder in an area subject to an AD inspection for cracks. The most recent AD inspection of the cylinder was performed 218 landings prior when the fatigue crack was large enough to be detectable. A previous AD inspection performed 497 landings prior to the accident also did not detect the crack that would have been marginally detectable at the time.

After the accident, World Atlantic Airlines performed a fleet campaign to reinspect all MD-80 MLG cylinders per the AD, retrained their company and contract personnel on the AD accomplishment procedures, and revised their task card procedures for the MLG inspection. They also revised the procedures for the slide inspections to ensure that the gas reservoir pressure was checked more regularly during service inspections and prior to each flight. The fractured RMLG (part number 5935355-501, serial number S1017) was installed on the accident airplane on May 10, 2011 after being overhauled in April 2011. Boeing released alert service bulletin (SB) MD80-32A344 on March 31, 2003, with instructions for performing fluorescent particle and fluorescent magnetic particle inspections of the MLG cylinders to detect cracks that could lead to fracture of the cylinder. The SB went through several revisions with revision 5 as the most current that was released on December 20, 2006. The Federal Aviation Administration (FAA) mandated the inspection of the MLG cylinders on MD-80 series airplanes per the procedures in the Boeing SB in Airworthiness Directive (AD) 2004-05-03 that became effective on March 15, 2004. The AD required repetitive inspections of the MLG cylinders at intervals not to exceed 450 landing cycles. The accident RMLG cylinder was last inspected per the AD on November 16, 2017, which was 218 landing cycles prior to the accident landing and 279 landing cycles since its previous inspection. Previous AD inspections of the RMLG cylinder occurred on March 30, 2017 (294 cycles since previous), November 1, 2016 (427 cycles since previous), and February 29, 2016 (414 cycles since previous). The L1 Escape Slide (part number D29982-121, serial number 1091) was installed on the accident airplane on January 25, 2018. According to the operator's maintenance program, the slides should be visually inspected every 3 days which includes a check of the bottle pressure. There were 22 service checks performed on the airplane between the slide installation date and the accident date and there were no non-routine maintenance actions performed on the L1 Escape Slide. Examination of the slide after the accident revealed that the release cable had been pulled, the inflation valve was open, and the pressure gauge on the reservoir and valve assembly (RVA) was showing zero pressure. The takeoff, climb, cruise, and descent portions of the flight were uneventful. The first officer was the pilot flying for the visual approach and landing on runway 14 at KAEX. Examination of the flight data recorder (FDR) data showed that the airspeed, attitude, and descent rate for the landing were similar to previous landings. After landing, the right main landing gear (RMLG) cylinder fractured and collapsed aft and the airplane settled onto the right wing and flaps. The crew was able to stop the airplane on the runway about 7,000 feet from the approach end and called for an emergencyevacuation due to a suspected fire on the right wing. The flight attendants opened the two forward and two aft doors for the evacuation. The escape slide at the left forward door (L1) did not deploy or inflate for the evacuation. The passengers and crew evacuated the airplane using the three doors with inflated slides. There were no injuries reported by the passengers or crew during the evacuation. The L1 Escape Slide was removed from the accident airplane and subjected to further examination at the manufacturer's facility under the direction of the NTSB Survival Factors Group. The examination found the slide assembly was in the fully packed condition with no evidence of gas flow into the assembly even though the inflation valve was open. The slide was found to inflate normally with a pressurized RVA. Further examination of the RVA found a slow leak through the rupture disk assembly on the inflation valve with three areas of surface corrosion on the sealing surface of the rupture disk assembly. Testing of the RVA revealed that the slow leak would empty the gas reservoir over a period of several months. The fractured RMLG was removed from the airplane and subjected to further examination at a Boeing facility in Huntington, Beach, CA, under the direction of the NTSB Materials Laboratory. The RMLG cylinder circumferential fracture was oriented perpendicular to the longitudinal axis of the RMLG about 30 inches from the bottom of the cylinder. Most of the fracture surface exhibited a dull, gray luster, with a rough surface texture and shear lips present on the edges of the fracture at the inner and outer diameter of the cylinder consistent with overstress separation. Chevron marks and river patterns on the fracture surface emanated from a small thumbnail crack on the forward side of the cylinder outer diameter. The thumbnail area was discolored, measured 0.072 inch in length and 0.035 inch in depth, and had crack arrest features and striations consistent with fatigue separation. There were no shear lips present at the thumbnail crack location. There was a rectangular area on the forward side of the cylinder around the thumbnail crack that measured about 4 inches longitudinally and 5 inches circumferentially that had a different surface finish. Examination in this area found primer and paint, no Cadmium plating on the base metal, and scratch marks consistent with mechanical grinding. There was some evidence of corrosion on the outer surface of the cylinder near but not at the fatigue crack initiation point. The cylinder material composition, microstructure, and hardness were consistent with the manufacturer's specifications. There was no evidence of excessive mechanical damage due to grinding in the area of the fatigue crack. A fatigue analysis was performed to estimate how long the fatigue crack was present in the cylinder. Fatigue striation counts were performed at 14 unique areas in the thumbnail area using a Scanning Electron Microscope (SEM).Each counted fatigue striation was assumed to correlate directly to a landing cycle. The data for this fatigue crack with a depth of 0.035 inch showed an estimated life of 1774 landing cycles. The data was used to estimate the crack length at the time of the most recent AD inspection 218 cycles prior to the accident and the previous inspection. The crack was estimated to have been 0.059 inch long at the time of the November 16, 2017 inspection and 0.048 inch long at the time of the March 30, 2017 inspection. The minimum detectable crack length stated in the SB was 0.050 inch which was confirmed by inspection personnel. The airplane had abrasion and impact damage to the right wingtip, right outboard slat, right outboard lower wing skin, right outboard flap, and the flap hinges. The navigation light lens on the right wingtip was shattered. The RMLG collapsed aft into the right inboard flap and side-of-body-fairing damaging both.